Compared with conventional magnetic recording systems, a non-contact optical recording system capable of recording and reproduction is characterized by resistance to scratches and stains and has been making a great contribution to large memory.
A recording medium of this system is composed of a transparent substrate made of, e.g., a polycarbonate resin, having a recording layer formed thereon. A polycarbonate resin is suitable as a material of the substrate for its high resistance to heat of melt molding, small dimensional change after molding, and excellent mechanical characteristics. Substrates for optical recording media should of necessity possess the characteristics required of a substrate, such as birefringence and mechanical characteristics, and are also required to be free from molding defects, such as pit deviations and mold release unevenness, which may be developed in the production of substrates. In the production of the substrate, a polycarbonate resin is usually molded in a higher temperature than for other purposes because a transparent substrate should be free of residual birefringence. Low-molecular weight volatile matter generated during the high temperature molding is apt to adhere to the stamper or the replica, causing a serious problem of servo control errors. How to reduce the volatile content has therefore been a big problem awaiting for a solution.
Of literature on polycarbonate resins suited for this use, there are many reports on low-molecular weight substances contained therein. For example, methods for diminishing low-molecular weight compounds by adding improvements to the polymerization technique are disclosed in JP-B-6-23243 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-6-336522 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and JP-A-3-109420. With regard to removal of low-molecular weight compounds from a produced polymer, JP-A-63-278929, JP-A-64-6020, and JP-A-4-306227 teach acetone extraction.
Existing literature, inclusive of the above, relates to how to reduce the low-molecular weight compound content of polycarbonate resins, with no considerations given to what kind of and how low-molecular weight compounds volatilize. Obviously, there is room for due consideration as regards these points. In particular, the means proposed for reduction of low-molecular weight components, such as acetone extraction, achieve relatively satisfactory effects but, in turn, necessitates a complicated step therefor (such as acetone extraction), which creates another issue in industrial production. For example, adoption of acetone extraction involves subsidiary steps for acetone separation and recovery as well as the step of acetone extraction itself, making the whole process considerably complicated.